The present invention relates to an end-point detector for a plasma etcher, which spectrometrically measures plasma light produced in a plasma etching (dry etching) step in a semiconductor-device manufacturing process, detects a time-based variation of measured spectrum intensity, and controls an etching end time point.
In general, in a plasma etcher, a reaction or process gas is introduced into a chamber and a radio-frequency voltage is applied between an upper electrode and a lower electrode serving also as a susceptor within the chamber at a predetermined distance therebetween. Thereby, a plasma of a process gas is produced between both electrodes and a fine pattern is formed by etching a semiconductor wafer or a film formed thereon placed on the susceptor.
Precise end-point detection by the plasma etcher is presently required, for example, in a step of etching an SiO.sub.2 film for formation of a contact hole or holes.
In the step of etching the SiO.sub.2 film, as shown in FIG. 8, an Si wafer 3, on which an SiO.sub.2 film 2 covered with a resist 1 having an opening 1a with a predetermined pattern is provided, is set on a susceptor. Then, while a mixture gas of CF.sub.4 and Ar is supplied as a process gas into the chamber, a radio-frequency voltage is applied between the electrodes. As a result, the following reaction occurs on an exposed portion of the SiO.sub.2 film through the opening 1a: EQU CF.sub.4 .uparw.+SiO.sub.2 .fwdarw.SiF.sub.4 .uparw.+CO.sub.2 .uparw.
Thus, SiO.sub.2 is etched.
In this case, there is a problem in the control of reaction time. If the reaction time is too short, the exposed portion of the SiO.sub.2 film is not completely removed and is left, as indicated by a broken line A. If the reaction time is too long, etching progresses deep into the region of SiO.sub.2 film which is covered with the resist, as indicated by a broken line B. This state is so-called "over-etching". Such defects in etching will seriously degrade the quality of products.
In the prior art, in order to solve the above problems, there is known a method of monitoring a spectrum intensity of a gas produced by the reaction of SiO.sub.2 (CO.sub.2 gas in this case), thereby controlling the reaction time. The spectrum intensity of CO.sub.2 gas varies with the passing of the reaction time, as shown in FIG. 9 by a solid line. The time of variation in spectrum intensity of CO.sub.2 gas in this case comprises a time period in which the intensity gradually decreases from the beginning of etching with progression of etching, a time period in which the intensity sharply decreases near an end point of etching, and a time period in which the intensity gradually until the occurrence of over-etching state. Accordingly, if the reaction time is controlled at around the time point at which the spectrum intensity sharply varies, the optimal etching state is attained. In the prior art, plasma light within the chamber is thus measured by the spectrometer and the spectrum intensity of CO.sub.2 gas is measured by a photodetector, and the reaction time is controlled at around the time point at which the spectrum intensity sharply varies.
With a recent development in miniaturization of semiconductor devices, the size of an opening 1a in resist 1 on wafer 3 shown in FIG. 8, i.e. a width of an etching pattern, has been decreased to a sub-micron order. In addition, an opening ratio of the opening 1a to the entire area of wafer 3 has been remarkably decreased from about 10% in the prior art to 5% and further to 1% or less. The decrease in the opening ratio means a decrease in variation of emitted-light spectrum due to CO.sub.2 gas in the above-described example of etching with SiO.sub.2. Consequently, it is difficult to exactly detect the end point in the above-described measuring technique.
For example, Jpn. Pat. Appln. KOKAI Publication No. 5-62943 and Jpn. Pat. Appln. KOKAI Publication No. 7-321094 disclose prior-art techniques of removing a drift of whole plasma light and improving S/N of signal, thereby enhancing sensitivity of detection.
In order to fully obtain the advantage of such techniques, it is important how to efficiently introduce the plasma light within the chamber into the spectroscope.
These prior-art documents describe that the efficiency of convergence of light to the spectroscope is enhanced by using a converging lens. FIGS. 10A, 10B and 11 illustrate how to use the converging lens for enhancing the efficiency of light convergence. Suppose that plasma light produced in a narrow space (a plasma producing region) between the upper electrode 7a and lower electrode 7b is represented by a horizontally elongated planar light source 4 (i.e. by dimensions of cross section of the plasma producing region). An incidence slit of the spectroscope is denoted by numeral 5.
FIG. 10A shows a case where no converging lens is provided. A light beam covering an effective numeral aperture NA (NAm) of the spectroscopic element (e.g. a concave grating) of the spectroscope through one point of the incidence slit 5 has to be collected from all point light sources within a circle 4a indicated on the planar light source 4. However, if the width of the planar light source 4 (i.e. the distance between the electrodes) is increased more than necessary, such an increased width will adversely affect the plasma process. Thus, the width is limited. Consequently, the amount of light in areas within the circle 4a projecting up an down from the width of the planar light source 4 becomes deficient. On the other hand, FIG. 10B shows a case where a converging lens 6 is provided between the planar light source 4 and incidence slit 5. In this case, if the converging lens having a numerical aperture enough to cover the effective numeral aperture of the spectroscopic element and the converging lens is situated such that an image of the incidence slit 5 having a width h and a length S1 may fall within a predetermined light strip 4b, all points of the incidence slit can pass a light beam of a numerical aperture enough to the spectroscopic element and the efficiency of light convergence to the spectroscopic device can be enhanced. This technique, however, is not sufficient.